Early concepts The idea of an underwater rail tunnel traversing San Francisco Bay from
Oakland Point was suggested by the San Francisco eccentric
Emperor Norton in a proclamation that he issued on May 12, 1872. Emperor Norton issued another proclamation on September 17, 1872, threatening to arrest the city leaders of Oakland and San Francisco for neglecting his earlier proclamation. Official consideration to the idea was first given in October 1920 by Major General
George Washington Goethals, the builder of the
Panama Canal. The alignment of Goethals's proposed tube, from the foot of
Market St. to the
Oakland mole, was almost exactly the same as today's Transbay Tube. His proposal called for building on bay mud, which anticipated some of the seismic design aspects of the finished Transbay Tube, and was estimated to cost up to . A competing bridge-and-tunnel proposal was advanced in July 1921 by J. Vipond Davies and
Ralph Modjeski, closer to the alignment of a proposed
Southern Crossing, between Mission Rock and
Potrero Point in San Francisco due east to
Alameda. Davies and Modjeski were critical of the ventilation issues that would arise from a long combined automobile and rail tunnel, indirectly endorsing the idea of a dedicated tunnel for electric rail traffic. The Davies and Modjeski proposal was joined by twelve other proposed projects to cross the Bay in October 1921, several of which featured rail service through long tunnels. In 1947, a joint Army-Navy Commission recommended
an underwater tube as a means of relieving automobile congestion on the then-ten-year-old
Bay Bridge. The recommendation was issued in a report undertaken to determine the feasibility of the
Reber Plan.
Construction Seismic studies commenced in 1959, including boring and testing programs in 1960 and 1964, and the installation of an earthquake recording system on the Bay floor. The tube's route was modified after preliminary surveys were unable to identify a continuous bedrock profile, requiring more precise boring and probing of the Bay floor. The route was deliberately chosen to avoid bedrock as much as possible so the tube was free to flex, avoiding concentrated bending stresses. Design concepts and route alignment were completed by July 1960. A 1961 report estimated the cost of the Transbay Tube at . Construction was started on the tube in 1965, and the structure was completed after the final section was lowered on April 3, 1969. Prior to being fitted out, the tube was opened for visitors to walk through a small portion on November 9, 1969. The tracks and
electrification needed for the trains were finished in 1973, and the tube was opened to service on September 16, 1974, five years after the originally-projected completion date, after clearing California Public Utilities Commission concerns regarding the automated dispatch system. The first test run was performed by a train under automatic control on August 10, 1973. Train No. 222 ran from to in seven minutes at and returned in six minutes at the full speed of , carrying approximately 100 passengers including BART officials, dignitaries and reporters. The tunnel is set in a trench wide with a gravel foundation deep. Lasers were used to guide the dredging of the trench and the laying of the gravel foundation, maintaining route accuracy of within for the trench and for the foundation. Construction of the trench required dredging of material from the Bay. The structure is made of 57 individual sections that were built on land at the
Bethlehem Steel shipyard on
Pier 70 and towed out into the bay by a large catamaran
barge. After the steel shell was completed, water-tight bulkheads were fitted and concrete was poured to form the -thick interior walls and track bed. They were then floated into place (positioned above where they were to sit), and the barge was tethered to the Bay floor, acting as a temporary tension leg platform. The section was ballasted with of gravel before being lowered into a
trench packed with soft soil, mud, and gravel for leveling along the Bay's bottom. Once the section was in place, divers connected the section with the sections that had already been placed underwater, the
bulkheads between placed sections were removed and a protective layer of sand and gravel was packed against the sides. with $90 million of that cost being spent on construction, the remainder going towards laying rails, electrification, ventilation and train control systems.
Configuration The western terminus of the Tube directly connects to the downtown
Market Street subway near the
Ferry Building, north of the Bay Bridge. The tube crosses under the western span of the Bay Bridge between the
San Francisco Peninsula and
Yerba Buena Island, and emerges in
Oakland along 7th Street, west of
Interstate 880. The tube has 57 sections; each section ranges from long. To conform with the route, 15 tube sections were curved horizontally, 4 were curved vertically, 2 had horizontal and vertical curves, and the remaining 36 sections were straight. The steel shell is thick, The tunnels are vented to the atmosphere at the San Francisco and Oakland ends and are vented to each other (through the upper gallery) with remotely-operated dampers long by high over every third door. which allows six degrees of freedom (translation along and rotation about three axes). As designed, the joints allow movement of up to along the tube's axis and up to vertically or laterally. A restaurant was constructed atop the San Francisco transition structure (vent) on a pier behind the Ferry Building. The Oakland vent structure is located in the middle of a
Port of Oakland container yard.
Seismic retrofitting The Transbay Tube has required
earthquake retrofitting, both on its exterior and in the interior. The total cost of seismic retrofits was estimated at in 2004. However, settling of the tube within its trench and the Loma Prieta quake had reduced the allowable movement of the seismic joints to as little as . The 1991 study was followed by a more detailed
BART Seismic Vulnerability Study, published in 2002, which concluded the fill packed around the tube might be prone to
soil liquefaction during an intense earthquake, which could allow the buoyant hollow tube to break loose from its anchorages or cause movement that would exceed the capacity of the sliding seismic joints. Retrofitting work required the fill to be compacted, to make it denser and less prone to liquefaction. Compaction started in Summer 2006 at the east end of the tube, on property belonging to the Port of Oakland. A 2010 paper concluded the distance the tube would rise due to liquefaction was limited based on model testing of potential liquefaction mechanisms, and questioned the justification for the compaction effort. On the interior of the tube, BART began a major retrofitting initiative in March 2013, which involved installing heavy steel plates at various locations inside the tube that most needed strengthening, to protect them from sideways movement in an earthquake. A vehicle was custom-built to handle the , thick plates; once hoisted in place, the plates were bolted to the existing concrete walls and welded together, end-to-end. The contract for was awarded to California Engineering Contractors for installation. In order to complete this work during 2013, BART closed one of the two bores of the tube early midweek (Tuesdays, Wednesdays and Thursdays), resulting in delays of 15–20 minutes. The work, originally estimated to last approximately 14 months, was completed by December 2013, after only 8 months of construction. In December 2016, BART awarded a contract to perform further seismic retrofitting. In this phase, a new steel liner and higher-capacity pumps would be installed to reduce the possibility of flooding the tube, as the existing pumps would not be adequate in the worst-case seismic event. Work was projected to start in the summer of 2018 and is scheduled to take more than two years to complete. Service through the tube would be reduced or eliminated during the first hour and the last three hours of the service day. == Incidents and issues ==